Bending oscillations are caused by rolling-off of the region of the axial groove or slot on the blanket cylinder against another cylinder. All measures known heretofore to reduce, or to suppress such bending oscillations were directed to reducing the size of the surface, or circumference, with respect to circular form, in the region adjacent the edges of the tensioning grooves or slots of the cylinders. The general concept was to cause the forces which arise upon engagement of the blanket stretched on the cylinder with a counter cylinder to increase gradually. The rise in engagement force, upon build-up of the force at the impression line between the rubber blanket and a counter cylinder, thus, should have been gradual after the groove had been passed during the roll-off of the rubber cylinder against another cylinder. Fed. Rep. German Patent No. 1,139,516 and German Democratic Patent No. 101,335 illustrate such arrangements, in which the circumference of the cylinder, adjacent the groove, is depressed with respect to a theoretical circle having the axis of rotation of the cylinder as a center.
Flattening, or chording the contour of the cylinder results in loss of engagement of the rubber blanket with another cylinder against which it rolls off, for simplicity, hereinafter, referred to as the "counter" cylinder. This counter cylinder may be a plate cylinder or another rubber blanket cylinder, or another cylinder within the printing machine. In the ordinary course of rotation of such a flattened or chorded cylinder, and particularly when two similar rubber blankets roll off against each other, the rubber blankets are first in engagement and, as they roll towards the flattened or chorded surface, the rubber blankets will lose contact, in other words, the impression force, or engagement force of a rubber blanket on one of the cylinders with respect to a rubber blanket on a counter cylinder becomes zero. Within the circumferential range in which the engagement force is small, or zero, the blanket cylinder, or cylinders tend to start to oscillate about their balanced position which they tend to assume when the rubber blanket is out of contact with a counter cylinder. The speed of current modern printing machines may be in the order of between 35,000 to 45,000 revolutions per hour (not quite 600 to about 750 rpm). Depending on the speed of the machine, the engagement forces against the blanket, after there was loss of contact or loss of engagement force, will occur in the form of a pulse-like sharp impact. These impacts occur when, after rolling over the clamping grooves, the engaged adjacent cylinders again come into contact as the surfaces thereof are again circular about the axis of rotation of the respective cylinder. This pulse or shock has been termed the engagement shock. The time at which the engagement shock occurs is of substantial importance with respect to the bending dynamics of the cylinder. The highest bending dynamics occur when the engagement shock arises precisely at the lower reversal, or nodal point of a cylinder oscillation. Rotary offset printing machines having cylinder diameters of about 20 cm, with customary clamping groove widths X of between about 6 to 8 mm, result in maximum oscillatory deflections just at the most customarily used upper speed ranges of the machines in which the cylinders are installed, that is, in the range of between 40,000 to 45,000 revolutions per hour (667 to 750 rpm). The oscillations of the cylinder after the engagement shock result in striping of the printed product which may be more or less visible, and, if noticeable, is of course undesired.